Trump's former chief of staff, John Kelly, sides with John Bolton and says the Senate should call witnesses in impeachment trial

President Donald Trump's former chief of staff, John Kelly, said he believes former national security adviser John Bolton's allegations concerning Trump's Ukraine pressure campaign.

Bolton wrote in his forthcoming book that the president told him last year he would withhold military aid to Ukraine until the Ukrainian president acceded to his demands for investigations into his political rivals.

Bolton's allegations contradict Trump's repeated claim that he didn't leverage the military aid to pressure Ukraine to investigate former Vice President Joe Biden and other Democratic opponents.

"If John Bolton says that in the book I believe John Bolton," Kelly, a retired general who previously served as Trump's secretary of homeland security, told a Florida audience on Monday night.

Kelly described Bolton as honest and said he supports the Senate calling witnesses in its impeachment trial — something the president and Republican leaders have staunchly opposed.

"Every single time I was with him ... he always gave the president the unvarnished truth," Kelly said of his former White House colleague.

Kelly's comments are an astonishing rebuke of the president, for whom he served as a close West Wing adviser for 18 months.